Line-source loudspeakers



Jan. 17, 1967' 1.. KLEPPER LINE-SOURCE LOUDSPEAKERS Filed July 24, 1963 FIG.

' FIG. 2

f-aoo c s 850 c s 400 CPS 850 CPS 2400 CPS 2400 CPS l 6800 CPS I\ 6800 CPS FIG. 4

FIG. 3

DAVID L.KLEPPER, INVENTOR.

BY M I M M ATTORNEYS United States Patent ()fiice 3,299,2bfi Patented Jan. 17, 1967 3,299,206 LINE-SOURCE LOUDSPEAKERS David L. Klepper, Cambridge, Mass., assignor to Bolt Beranek and Newman Inc., Cambridge, Mass., a corporation of Massachusetts Filed July 24, 1963, Ser. No. 397,359

Claims. (Cl. 179-1) The present invention relates to a line-source or column loudspeaker array comprising a plurality of individual speaker units disposed in a line substantially orthogonal to the radiation direction of the individual speaker units.

Line-source loudspeaker arrays have recently become of great interest in the audio reproduction field, for example, because such arrays, which may be easily assembled of low-priced, cone-type loudspeakers, provide a high degree of directivity in a plane including the line along which the individual cone-type loudspeakers are arranged. Dramatic improvements in speech intelligibility have been achieved using such an array by cutting through roomreverberation effects, with particular improvements along the major lobe of the acoustic directivity transmission pattern. The width of the major lobe, however, narrows as the frequency of the transmitted signal is increased and, in addition, side lobes at higher frequencies are particularly troublesome. This is caused because the length or aperture of the array is only a few low-frequency wavelengths, but is a large number of high-frequency wavelengths. The width of the major lobe may be maintained substantially constant over a band of transmitted acoustic signal frequencies only by maintaining the effective length of the array, in wavelengths, substantially constant over the frequency range of interest. This may be accomplished, for example, by employing filters to attenuate higher frequencies of the electric signals in the outer speakers of the array; but the improvement thereby attained is small. It has also been proposed to skew the speakers along the line of the array, but this, also, has limited effect.

An object of the present invention, accordingly, is to provide a new and improved line-source loudspeaker array that is not subject to the before-mentioned limitations, but which, to the contrary, has a directivity pattern of acoustic transmission that is substantially constant over the frequency band of interest.

Another object is to provide an array for use in the field of audio reproduction and in which the characteristics before-mentioned are attainable without detracting substantially from the simplicity or adding substantially to the cost of available line-source loudspeakers.

Still another object is to provide a novel loudspeaker array of more general utility, also.

Still other objects will be evident in the description to follow and will be particularly pointed out in connection with the appended claims.

Generally, and by way of summary, the objects of the invention are attained in a line-source loudspeaker array of combined low-, medium-, and high-sound frequency transmitting devices disposed along a line, the Width of the transmission directivity pattern of which at the low-, medium-, and high-frequencies would normally be markedly different. The array is modified to render such directivity patterns substantially similar over the frequency range of interest by disposing along the array means having high-frequency-absorbing properties that successively increase from the center of the array along the said line toward the far ends thereof, whereby the effective length of the array, in wavelengths, as modified by the absorbing means, at the low-, medium-, and highsound frequencies is rendered substantially the same.

The invention will now be described in connection with the accompanying drawing, FIG. 1 of which is a longitudinal sectional view, partially cut away and partially in schematic form, of a line-source speaker array embodying the inventive concept herein described; I

FIG. 2 is a fragmentary isometric view, partly cut away, showing the alignment of the individual speakers comprising the array of FIG. 1;

FIG. 3 is a plot of the main-lobe directivity patterns of the array at different excitation frequencies, operated in accordance with the prior art; and

FIG. 4 is a plot similar to that in FIG. 3 of the mainlobe directivity patterns for an array embodying the present invention.

Referring to FIGS. 1 and 2, a line-source speaker array is shown comprising a plurality of individual speaker units 1 disposed substantially in a line and contained within an appropriate enclosure 3. The speakers 1 are adapted to receive electric signals from a signal source 4, as is well known, the electric signals passing in a simple series-parallel arrangement, through conductors 5-6 and 5'6 to the said speakers.

The speakers 1, upon being energized by the electric signals, covering a predetermined band or range of loW-, medium-, and high-sound frequencies, will normally transmit acoustic Waves which have vastly different-width directivity patterns (at half power or half voltage points), in a plane through the line of the array, for the said low-, medium-, and high-sound frequencies. In FIG. 3, therefore, the directivity pattern for a high 6800-cycle acousticwave frequency has a much narrower major lobe than does the directivity pattern for a low 400-cycle acoustic wave in the illustrative example of a 5 /2-foot line-source speaker array. The array, thus, is relatively highly directional for high frequencies but not for low frequencies. As may be appreciated, a listener will, therefore, obtain a different mixture of acoustic frequencies if seated to the right or left of the major lobe than if located at or Within the region of its radiation axis.

There exist absorbing materials, such as glassfiber of the PF-lOS type, manufactured by Owens-Corning Fiberglas Corporation, that appreciably absorb the high acoustic frequencies but are substantially transparent to the lower frequencies. By placing the thin edges of wedges 7 and 8 of such absorbing material near the center of the array, to present a slight thickness of absorbing material near the center, and placing successively thicker portions of the wedges of increasing attenuation at successive regions toward the far ends of the array, the array is effectively shortened at the high frequencies but little affected at the low frequencies by the presence of the said wedges. Thus, by controlling the rate of change of thickness of the wedges toward the said far ends, the array may be made to have an effective length (in wavelengths) that remains constant for all frequencies in the range of interest. Properly chosen glassfiber wedges 7 and 8, ranging up to 3 inches in thickness, for example, resulted in the directivity patterns shown in FIG. 4; the width of the major lobe of the 6800-cycle acoustic wave being very close to, or more substantially the same as, that of the 40G-cycle acoustic wave. It has been found, in addition, that the wedges 7 and 8 reduce the undesirable side lobes at the higher frequencies.

Modifications will occur to those skilled in the art without departing from the inventive concept herein disclosed and defined in the appended claims.

What is claimed is:

1. An array of combined low-, medium-, and highsound frequency transmitting devices the width of the transmission directivity patterns of which at the low-, medium-, and high-frequencies are markedly different, having apparatus for rendering such directivity patterns substantially similar, that comprises, means disposed along the array of high-frequency-absorbing properties that successively increase from one portion of the array to another, whereby the effective length of the array, in wavelengths, as modified by the absorbing means, and the directivity patterns at the low-, medium-, and high-sound frequencies are rendered more substantially the same.

2. An array of combined low-, medium-, and highsound frequency transmitting devices the width of the transmission directivity patterns of which at the low-, medium-, and high-frequencies are markedly different, having apparatus for rendering such directivity patterns substantially similar, that comprises, acoustic means disposed along the array of high-frequency-absorbing properties that successively increase from the center of the array toward the far ends thereof, whereby the effective length of the array, in wavelengths, as modified by the absorbing means, and the directivity patterns at the low-, medium-, and high-sound frequencies are rendered more substantially the same.

3. An array of combined low-, medium, and highsound frequency transmitting devices the width of the transmission directivity patterns of which at the low-, medium-, and high-frequencies are markedly different, having apparatus for rendering such directivity patterns substantially similar, that comprises, glassfiber wedge means disposed along the array of high-frequency-absorbing properties that successively increase from one portion of the array to another, whereby the effective length of the array, in wavelengths, as modified by the absorbing means, and the directivity patterns at the low-, medium-, and high-sound frequencies are rendered more substantially the same.

4. An array as claimed in claim 3 and in which the wedge means is disposed with the thin edges thereof near the center of the array.

5. In a line-source loudspeaker array to transmit a wide band of acoustic frequencies, and comprising a plurality of individual speaker units disposed in a line, the array having characteristic directivity transmission patterns in a plane including the said line that are markedly dififerent at low-, medium-, and high-sound frequencies, apparatus for rendering the said directivity patterns substantially similar comprising a pair of wedge-shaped acoustic absorbers, having high-frequency absorbing properties, disposed along the said line with the thin edges of the wedges disposed near the center of the array and of increasing thickness toward the far ends thereof, whereby the etfective length of the array in wavelengths, as modified by the wedges, at the low-, 1nedium-, and highsound frequencies is render more substantially the same.

References Cited by the Examiner Column Loudspeaker System, Electronics World, June 1963; pages 25-27 and 76.

KATHLEEN H. CLAFFY, Primary Examiner.

R. MURRAY, Assistant Examiner. 

1. AN ARRAY OF COMBINED LOW-, MEDIUM-, AND HIGHSOUND FREQUENCY TRANSMITTING DEVICES THE WIDTH OF THE TRANSMISSION DIRECTIVITY PATTERNS OF WHICH AT THE LOW-, MEDIUM-, AND HIGH-FREQUENCIES ARE MARKEDLY DIFFERENT, HAVING APPARATUS FOR RENDERING SUCH DIRECTIVITY PATTERNS SUBSTANTIALLY SIMILAR, THAT COMPRISES, MEANS DISPOSED ALONG THE ARRAY OF HIGH-FREQUENCY-ABSORBING PROPERTIES THAT SUCCESSIVELY INCREASE FROM ONE PORTION OF THE ARRAY TO ANOTHER, WHEREBY THE EFFECTIVE LENGTH OF THE ARRAY, IN WAVELENGTHS, AS MODIFIED BY THE ABSORBING MEANS, AND THE DIRECTIVITY PATTERNS AT THE LOW-, MEDIUM-, AND HIGH-SOUND FREQUENCIES ARE RENDERED MORE SUBSTANTIALLY THE SAME. 